push: allow delete single-level ref

We discourage the creation/update of single-level refs
because some upper-layer applications only work in specified
reference namespaces, such as "refs/heads/*" or "refs/tags/*",
these single-level refnames may not be recognized. However,
we still hope users can delete them which have been created
by mistake.

Therefore, when updating branches on the server with
"git receive-pack", by checking whether it is a branch deletion
operation, it will determine whether to allow the update of
a single-level refs. This avoids creating/updating such
single-level refs, but allows them to be deleted.

On the client side, "git push" also does not properly fill in
the old-oid of single-level refs, which causes the server-side
"git receive-pack" to think that the ref's old-oid has changed
when deleting single-level refs, this causes the push to be
rejected. So the solution is to fix the client to be able to
delete single-level refs by properly filling old-oid.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -1463,7 +1463,9 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 		find_shared_symref(worktrees, "HEAD", name);
 
 	/* only refs/... are allowed */
-	if (!starts_with(name, "refs/") || check_refname_format(name + 5, 0)) {
+	if (!starts_with(name, "refs/") ||
+	    check_refname_format(name + 5, is_null_oid(new_oid) ?
+				 REFNAME_ALLOW_ONELEVEL : 0)) {
 		rp_error("refusing to update funny ref '%s' remotely", name);
 		ret = "funny refname";
 		goto out;

connect.c

@@ -30,7 +30,8 @@ static int check_ref(const char *name, unsigned int flags)
 		return 0;
 
 	/* REF_NORMAL means that we don't want the magic fake tag refs */
-	if ((flags & REF_NORMAL) && check_refname_format(name, 0))
+	if ((flags & REF_NORMAL) && check_refname_format(name,
+							 REFNAME_ALLOW_ONELEVEL))
 		return 0;
 
 	/* REF_HEADS means that we want regular branch heads */

t/t5516-fetch-push.sh

@@ -401,6 +401,11 @@ test_expect_success 'push with ambiguity' '
 
 '
 
+test_expect_success 'push with onelevel ref' '
+	mk_test testrepo heads/main &&
+	test_must_fail git push testrepo HEAD:refs/onelevel
+'
+
 test_expect_success 'push with colon-less refspec (1)' '
 
 	mk_test testrepo heads/frotz tags/frotz &&
@@ -898,6 +903,13 @@ test_expect_success 'push --delete refuses empty string' '
 	test_must_fail git push testrepo --delete ""
 '
 
+test_expect_success 'push --delete onelevel refspecs' '
+	mk_test testrepo heads/main &&
+	git -C testrepo update-ref refs/onelevel refs/heads/main &&
+	git push testrepo --delete refs/onelevel &&
+	test_must_fail git -C testrepo rev-parse --verify refs/onelevel
+'
+
 test_expect_success 'warn on push to HEAD of non-bare repository' '
 	mk_test testrepo heads/main &&
 	(